This study presents the first description of social behavior and structure in native populations of the coypu (Myocastor coypus). Data from behavioral observations and trapping were used to describe group composition and behavior at 3 study areas in east-central Argentina in 1999–2000. Seven groups were described (11 coypus/group ± 1.5 SE). Groups were characterized by several adult and subadult females, 1 dominant male, several adult and subadult males, and a variable number of juveniles. Subgroups (mainly composed of juveniles and small adult males) were also identified within some groups. Group fidelity was relatively high. Amicable interactions and cooperative behaviors such as nursing in groups, allogrooming, and alarm calls occurred within groups. Agonistic interactions and marking behavior were rare but suggested that the largest male was socially dominant. This study confirms gregarious habits and a polygynous mating system for the species and provides the first report of the existence of some cooperative behaviors.